This invention relates to an Orthogonal Frequency Division Multiplex (OFDM) transceiver apparatus and, more particularly, to an OFDM transmitter apparatus and OFDM receiver apparatus for performing OFDM communication utilizing a plurality of polarized waves.
In wideband wireless communications, frequency-selective fading ascribable to a multipath environment occurs. A useful scheme for dealing with this is multicarrier modulation, which divides the transmission band into a plurality of narrow bands (carriers) and transmits them in parallel in such a manner that frequency-selective fading will not occur. At present, specifications relating to digital TV/audio broadcasting (Japan and Europe) and wireless LAN (IEEE 802.11a) are being standardized with the Orthogonal Frequency Division Multiplex (OFDM) transmission scheme, which is one type of multicarrier modulation scheme, as the base. Further, OFDM-based modulation schemes have been proposed even in next-generating mobile communication systems.
In FIG. 9, (a) is a schematic structural view of a multicarrier transmission scheme. A serial/parallel converter 1 converts serial data to parallel data and inputs the parallel data to quadrature modulators 3a to 3d via low-pass filters 2a to 2d, respectively. In the Figure, the conversion is to parallel data comprising four symbols S1 to S4. Each symbol includes an in-phase component and a quadrature component. The quadrature modulators 3a to 3d subject each of the symbols to quadrature modulation by carriers having frequencies f1, to f4 illustrated in (b) of FIG. 9, a combiner 4 combines the quadrature-modulated signals and a transmitter (not shown) up-converts the combined signal to a high-frequency signal and then transmits the high-frequency signal. With the multicarrier transmission scheme, the frequencies are arranged, as shown at (b) of FIG. 9, in such a manner that the spectrums will not overlap in order to satisfy the orthogonality of the carriers.
In Orthogonal Frequency Division Multiplex (OFDM) transmission, frequency spacing is arranged so as to null the correlation between a modulation band signal transmitted by an nth carrier of a multicarrier transmission and a modulation band signal transmitted by an (n+1)th carrier. FIG. 10(a) is a diagram of the structure of a transmitting apparatus that relies upon the OFDM transmission scheme. A serial/parallel converter 5 converts serial data to parallel data comprising M-number of symbols. An IFFT (Inverse Fast Fourier Transform) 6, which is for the purpose of transmitting the M-number of symbols as carriers having a frequency spacing shown at (b) of FIG. 10, applies an inverse discrete Fourier transform to the frequency data to effect a conversion to time data. A guard interval inserting unit 7 inserts a guard interval GI and inputs real and imaginary parts to a quadrature modulator 9 through low-pass filters 8a, 8b. The quadrature modulator 9 subjects the input data to quadrature modulation, and a transmitter (not shown) up-converts the modulated signal to a high-frequency signal. In accordance with an OFDM transmission scheme, a frequency placement of the kind shown at (b) of FIG. 10 becomes possible, thereby enabling an improvement in the efficiency with which frequency is utilized.
FIG. 11 is a diagram showing the conventional structure of an Orthogonal Frequency Division Multiplex (OFDM) communication apparatus, in which TR and RV represent transmit and receive channels. On the transmit channel TR, a serial/parallel (S/P) converter 10 converts transmit data, which enters in a serial format, to M-bit parallel data, and a mapping unit 11 maps the M-bit parallel data to N-number of carriers based upon a modulation scheme of each carrier. For example, if it is assumed that QPSK modulation is performed by all carriers, the M-bit parallel data is divided into N-sets of two bits each and the 2-bit data of the N sets obtained by division is mapped to each carrier. FIG. 12 is a diagram useful in describing carrier placement. Here carriers for transmitting a pilot have been inserted.
FIG. 13 is a diagram of signal-point placement for describing mapping. Here (a) is a case where BPSK modulation is performed, and one bit (b0) at a time is mapped to a carrier; (b) is a case where QPSK modulation is performed, and two bits (b0b1) at a time are mapped to a carrier; (c) is a case where 16 QAM modulation is performed, and four bits (b0b1b2b3) at a time are mapped to a carrier; and (d) is a case where 64 QAM modulation is performed, and six bits (b0b1b2b3b4b5) at a time are mapped to a carrier.
Returning to FIG. 11, an IFFT arithmetic unit 12 applies IFFT processing to the symbol data of N carriers to convert the data to two time waveform signals (PCM waveform signals) of a real number (Ich component) and imaginary number (Qch component). A guard interval inserting unit 13 inserts a GI (Guard Interval) into each signal, and a waveshaping unit 14 shapes the waveforms and inputs the results to an IQ modulator (QPSK quadrature modulator) 15. The latter applies quadrature modulation to the Ich signal and Qch signal input thereto, and a mixer 16 multiplies the modulated signal of the baseband by a high-frequency carrier wave that enters from a carrier wave generator 17, thereby performing a frequency conversion. A transmit amplifier 18 amplifies the transmit signal and transmits the amplified signal from an antenna ATT.
On the receive channel RV, a high-frequency amplifier 20 of a radio unit amplifies a receive signal from an antenna ATR, and a mixer 21 multiplies the receive signal by a high-frequency carrier wave that enters from a carrier wave generator 22, thereby effecting a frequency conversion to a baseband signal, and inputs the signal to an IQ demodulator (QPSK quadrature demodulator) 23. The latter subjects the input signal to quadrature demodulation to demodulate and output the Ich signal and Qch signal. A waveshaping unit 24 shapes each of the waveforms and inputs the results to a rotator 25. The latter detects phase-error information from a known pilot signal and rotates phase in such a manner that the phase error becomes zero. A GI removing unit 26 removes the GI (Guard Interval) from the input signal of each component, and an FFT arithmetic unit 27 applies FFT processing to the time waveform signals input thereto and outputs N-number of carrier components. A demapping unit 28 performs demapping (processing that is the reverse of mapping) on a per-carrier basis and outputs M-bit parallel data, and a parallel/serial (P/S) converter 29 converts the M-bit parallel data to serial data and outputs the serial data.
A communication method having a maximum communication speed of 54 Mbps stipulated by IEEE 802.11b (ARIB STD T71) can no longer be deemed satisfactory when one considers a communication environment such as that of present-day wireless LANs. Meanwhile, the radio band of less than 5 GHz is already saturated and the situation is such that dedicated bandwidths of frequency cannot readily be enlarged. A wired LAN generally is implemented according to 100Base-TX (100 Mbps), and a wireless LAN system having a communication speed equivalent to this is required.
Methods of enlarging transmission capacity without changing the frequency band include a co-channel transmission method. According to co-channel transmission, polarization (horizontal polarization and vertical polarization) of radio waves is changed to perform selective communication in the same frequency band, and transmission speed is doubled overall, as illustrated in FIG. 14. In order to perform OFDM transmission by the co-channel transmission method, two OFDM transceivers are required. Cross-polarization interference occurs owing to a shift in antenna polarization angle. Further, cross-polarization interference occurs also owing to distortion in the transmission path conforming to rainfall and other factors. This makes it necessary to construct an OFDM transceiver in such a manner that such cross-polarization interference can be eliminated.
Orthogonality holds between adjacent carriers in OFDM. Accordingly, a carrier CA and an adjacent carrier CB are always in an orthogonal relationship, as illustrated at (a) of FIG. 15, and do not interfere with each other. Further, although interference occurs between the carrier CA and a carrier CD, the carrier CA and a carrier CE are in an orthogonal relationship and interference between the polarized waves does not occur. Cross-polarization interference between the carrier CA and the carrier CD can be eliminated by interference compensation techniques using a conventional canceller.
However, when a phase deviation 0 occurs between a vertically polarized wave and a horizontally polarized wave, as shown at (b) of FIG. 15, owing to a shift in antenna polarization angle, etc., interference is produced between the carrier CA and the carrier CD and between the carrier CA and the carrier CE. This makes it necessary to eliminate the interference between the carriers CA and CD and the carriers CA and CE caused by the phase deviation.
Methods of interference compensation in a cochannel transmission scheme are disclosed in Japanese Patent Application Laid-Open No. 61-5642, Japanese Patent Application Laid-Open No. 5-48567 and Japanese Patent Application Laid-Open No. 6-181464. Further, OFDM transmission by a cochannel transmission scheme is disclosed in Japanese Patent Publication No. 8-504544 (Patent No. 3265578).
However, the interference compensating techniques disclosed in Japanese Patent Application Laid-Open No. 61-5642, Japanese Patent Application Laid-Open No. 5-48567 and Japanese Patent Application Laid-Open No. 6-181464, cannot eliminate interference between the carrier CA and the carrier CE caused by the phase deviation. Further, although Japanese Patent Publication No. 8-504544 (Patent No. 3265578) discloses that OFDM transmission is performed by a cochannel transmission scheme, there is no disclosure of a technique for eliminating interference between the carrier CA and the carrier CE.